Increased understanding of the role of carbohydrates as recognition elements on the surface of cells has led to increased interest in the production of carbohydrate molecules of defined structure. For instance, compounds comprising the oligosaccharide moiety, sialyl lactose, have been of interest as neutralizers for enterotoxins from bacteria such as Vibrio cholerae, Escherichia coli, and Salmonella (see, e.g., U.S. Pat. No. 5,330,975). Sialyl lactose has also been investigated for the treatment of arthritis and related autoimmune diseases. In particular, sialyl lactose is thought to inhibit or disrupt the degree of occupancy of the Fc carbohydrate binding site on IgG, and thus prevent the formation of immune complexes (see, U.S. Pat. No. 5,164,374). Recently, sialyl-α(2,3)galactosides, sialyl lactose and sialyl lactosamine have been proposed for the treatment of ulcers, and Phase I clinical trials have begun for the use of the former compound in this capacity. See, Balkonen et al., FEMS Immunology and Medical Microbiology 7:29 (1993) and BioWorld Today, p.5, Apr. 4, 1995. As another example, compounds comprising the sialyl Lewis ligands, sialyl Lewisx and sialyl Lewisa are present in leukocyte and non-leukocyte cell lines that bind to receptors such as the ELAM-1 and GMP 140 receptors. Polley et al., Proc. Natl. Acad. Sci., USA, 88:6224 (1991) and Phillips et al., Science, 250:1130 (1990), see, also, U.S. Ser. No. 08/063,181.
Because of interest in making desired carbohydrate structures, glycosyltransferases and their role in enzyme-catalyzed synthesis of carbohydrates are presently being extensively studied. The use of glycosyltransferases for enzymatic synthesis of carbohydrate offers advantages over chemical methods due to the virtually complete stereoselectivity and linkage specificity offered by the enzymes (Ito et al., Pure Appl. Chem., 65:753 (1993) U.S. Pat. Nos. 5,352,670, and 5,374,541). Consequently, glycosyltransferases are increasingly used as enzymatic catalysts in synthesis of a number of carbohydrates used for therapeutic and other purposes.
Carbohydrate compounds produced by enzymatic synthesis or by other methods are often obtained in the form of complex mixtures that include not only the desired compound but also contaminants such as unreacted sugars, salts, pyruvate, phosphate, PEP, nucleosides, nucleotides, and proteins, among others. The presence of these contaminants is undesirable for many applications for which the carbohydrate compounds are useful. Previously used methods for purifying oligosaccharides, such as chromatography, i.e., ion exchange and size exclusion chromatography, have several disadvantages. For example, chromatographic purification methods are not amenable to large-scale purifications, thus precluding their use for commercial production of saccharides. Moreover, chromatographic purification methods are expensive. Therefore, a need exists for purification methods that are faster, more efficient, and less expensive than previously used methods. The present invention fulfills this and other needs.